How to read input string from keyboard using assembly language

Question

I'm trying to write a simple program that takes string from the keyboard, then prints it to the screen. So far I couldn't make it work.

Here's the code:

    .section .rodata
output: .string "you entered %s\n"
input:  .string "%s"

    .text
.globl  main
    .type   main, @function

main:
    pushl   %ebp
    movl    %esp, %ebp

    subl    $100, %esp
    pushl   $input
    call    scanf

    movl    %ebp, %esp
    subl    $100, %esp
    pushl   $output
    call    printf

    xorl    %eax, %eax
    movl    %ebp, %esp
    popl    %ebp
    ret

When I execute it, the output is you entered (null) for any given input. when I set the offset of subl $100, %esp command (the one before call print ) to subl $104, %esp I get you entered %s , and when the offset is set to 108 I get you entered *gibberish* .

I feel like it's a game where I need to guess where scanf saved the string on the stack (why isn't it where it should be?).

I am using IA32 instruction set.

Any help would be greatly appreciated.

Answer 1

Basically there are 3 problems in your program:

subl    $100, %esp
pushl   $input
# Error 1:
# As Frank Kotler already wrote at this point
# only $input is stored on the stack; however
# the address of the buffer must also be on
# the stack (see Frank Kotler's comment)
call    scanf

movl    %ebp, %esp
# Error 2:
# Now the buffer is below ESP.
# Because interrupts use the kernel stack they
# will not overwrite the memory below ESP.
# However signals will destroy the memory below
# ESP!!
#
# Instead of the lines:
#   movl    %ebp, %esp
#   subl    $100, %esp
#
# You should use something like this:
#   add $8, %esp
# or:
#   lea -100(%ebp), %esp
#

subl    $100, %esp
# Error 3:
# As for "scanf" the second argument
# is missing on the stack
pushl   $output
call    printf

Answer 2

[org 0x0100]
jmp start
;=======Data===================================
s1: db 'Enter String: $'
s2: db 'Reversed String:$'
linefeed: db 10, '$'
;=======Read string and store into stack=======
start: mov dx, s1
mov ah, 09h
int 21h
mov dx, linefeed
mov ah, 09h
int 21h
mov si,0
mov bx,0
again: mov ah, 01h
int 21h
cmp al,byte 0dh
je endread
Page 8 of 10
mov bl,al
push bx
inc si
cmp si,15
je endread
jmp again
endread: Push si
;=======Read from stack and print to screen=====
mov dx, linefeed
mov ah, 09h
int 21h
mov dx, s2
mov ah, 09h
int 21h
mov dx, linefeed
mov ah, 09h
int 21h
pop si
r1: pop bx
mov dl,bl
mov ah, 02h
int 21h
dec si
cmp si,0
jne r1
mov dx, linefeed
mov ah, 09h
int 21h
mov ax, 0x4c00 ;terminate program
int 21h